1. FIELD OF THE INVENTION
The invention relates to an ultrasonic probe for measuring liquids at high temperature and under high pressure, and more particularly to ultrasonic probes designed to produce or to detect ultrasonic wave beams. Such ultrasonic probe are generally used for measuring speeds or flow rates of corrosive or noncorrosive liquids which circulate under pressure and at high temperature in piping.
2. DESCRIPTION OF THE PRIOR ART
Probes which may be used for measurements of the above type are known, but their use is limited to maximum temperatures of approximately +80.degree. C. and maximum pressures of approximately 100 bars.
Such a probe generally comprises a leak-tight monobloc cylindrical body or sleeve one of whose ends is closed by a thin wall which constitutes the front transmitting or receiving face for the ultrasonic waves. The thin wall should be leak-tight, able to withstand pressure and temperature, chemically inert with respect to the medium in which the probe is to operate and must also enable propagation of the ultrasonic beam in both directions with adequate transparency and good directional characteristics.
The French Pat. No. 2,150,630 discloses an ultrasonic probe for accurately measuring speeds and flow rates in piping in which corrosive liquids are circulating, in particular ergol used in liquid propellent rockets under pressures of 50 bars. The probe according to this French Patent comprises a protective sleeve and a single element which is in the form of a thick tube and closed at one end by a thin wall. The internal face of the tube is in electrical and mechanical contact with a piezoelectric plate, which is in contact with a damping cylinder. The sleeve completely protects the internal elements of the probe against the mechanical or corrosive actions of the measuring medium whilst enabling passage of the ultrasonic beams without prohibitive absorption.
A first difficulty encountered in manufacturing these probes is the machining of the thin wall whose thickness is approximately of 0.1 mm, and the parallelism of whose faces must be carried out accurately. Another difficulty is the mechanical and electrical connection of the active face of the piezoelectric plate contacting with the thin wall. In effect, piezoelectric material of the plate has been selected for such types of measurements as a result of its own properties, such as its sensitivity and its ability to support the high temperature of the probing medium.
For the accurate measurement of speeds and in particular low speeds, for example 1 cm/sec, and flow rates, a piezoelectric material having high sensitivity is selected. This condition is accomplished for the probe according to the above-mentioned French Patent. The piezoelectric plate is made of lead zirconate-titanate. The connection between the active face of the piezoelectric plate and the thin wall of the thick tube is constituted by a thin layer of solid conducting adhesive, such as an epoxy resin charged with silver powder. The connection attained in this way resists temperatures of up to 130.degree. C.; the probe may be used to a maximum temperature of 100.degree. C. and a pressure of approximately 100 bars.
In addition the French Pat. No. 2,063,324 discloses an ultrasonic probe for locating metallic parts immersed in a high temperature liquid metal. The ultrasonic probe comprises a transducing piezoelectric capsule made of lithium niobate and a damping cylinder which are curvated in a leak-tight metallic casing with thin front wall. The two faces of the piezoelectric capsule are coated with a thin silver coating then by a layer of copper deposited by electrolysis on the thin silver coating. The fixing of the two piezoelectric capsule faces onto the thin wall of the casing and onto the damping cylinder is carried out by silver brazing.
It is noted that a connection of this type is possible on account of the nature of the material constituting the piezoelectric capsule, i.e. the lithium niobate, whose Curie point is approximately 1200.degree. C. However the low sensitivity of this material, which is adequate for the above purpose, that is the location of immersed objects, is not suitable for the accurate measurement of liquid speeds or flow rates.
The problem of the accurate measurement of speeds or flow rates of liquids under pressure and at high temperatures by means of ultrasonic probes has therefore not been satisfactorily solved as the sensitive piezoelectric materials have a relatively low Curie point which prevents the use of brazing techniques, whereas piezoelectric materials having a high Curie point do not have adequate sensitivity.